SMPS wall wart failure.
 

"N_Cook" wrote in message ...

On 07/06/2015 04:41, David Farber wrote:
I have a Netgear cable modem. It is powered by a Jentec Technology
AF1205-B
power adapter. The adapter failed and I replaced it with another adapter
but
of course I was curious what the fault was. I opened up the case and found
a
bad (bulging) 1,000uF 10V filter cap. I did some other circuit tests to
make
sure there were no shorts because I was getting extremely low ESR readings
(less than 0.1 ohms) around the bad cap, Eventually I found an SMD zener
diode that was shorted. There are no identifying marks on the diode other
than its color which is the standard looking orange. I am fairly sure it's
a
zener because the pc board has it marked as ZD-2. Its location in the
circuit is in parallel with the 5 volt power supply diode that comes off
of
the secondary of the switching transformer. The power diode has two sets
of
numbers. The top row is marked 540 (perhaps an SB540?) on the bottom row
is
marked 849. That diode is ok. Only the zener is shorted. I was wondering
if
there was some ballpark zener voltage that I could use to replace the old
one. It's always good to have a spare 5V supply.

Thanks for your replies.


Could it be a VTS, ie designed to go short circuit in overvoltage
situation? Perhaps the ovelay component character set does not allow for
VTS .





I had to look that up.

You mean TVS.



Gareth.

SMPS wall wart failure.
 

On 07/06/2015 09:53, Gareth Magennis wrote:


"N_Cook" wrote in message ...

On 07/06/2015 04:41, David Farber wrote:
I have a Netgear cable modem. It is powered by a Jentec Technology
AF1205-B
power adapter. The adapter failed and I replaced it with another
adapter but
of course I was curious what the fault was. I opened up the case and
found a
bad (bulging) 1,000uF 10V filter cap. I did some other circuit tests
to make
sure there were no shorts because I was getting extremely low ESR
readings
(less than 0.1 ohms) around the bad cap, Eventually I found an SMD zener
diode that was shorted. There are no identifying marks on the diode other
than its color which is the standard looking orange. I am fairly sure
it's a
zener because the pc board has it marked as ZD-2. Its location in the
circuit is in parallel with the 5 volt power supply diode that comes
off of
the secondary of the switching transformer. The power diode has two
sets of
numbers. The top row is marked 540 (perhaps an SB540?) on the bottom
row is
marked 849. That diode is ok. Only the zener is shorted. I was
wondering if
there was some ballpark zener voltage that I could use to replace the old
one. It's always good to have a spare 5V supply.

Thanks for your replies.


Could it be a VTS, ie designed to go short circuit in overvoltage
situation? Perhaps the ovelay component character set does not allow for
VTS .





I had to look that up.

You mean TVS.



Gareth.

Brain-fart, I end up typing VTS a lot in another subject area of marine
flooding issues, standing for (Southampton) Vessel Traffic Services, and
also Voltage Transient sounds more appropriate than Transient Voltage